Bath composition for electropolishing of titanium and method for using same

ABSTRACT

A bath composition for the electropolishing of a metal surface made of nonalloyed titanium is disclosed. The bath composition may comprise sulfuric acid of 2 to 40% by volume, hydrofluoric acid of 10 to 18% by volume and acetic acid of 42 to 62% by volume.

FIELD OF THE INVENTION

The present invention relates to a bath composition for theelectropolishing of a metal surface made of nonalloyed titanium, and toa method of using this bath.

The term “polishing” is understood to mean a treatment for the purposeof reducing the roughness of a metal surface and, thereby, of increasingits brightness with, as a consequence, a lower corrosion sensitivity.

BACKGROUND OF THE INVENTION

Apart from the mechanical means used for this purpose (the use ofabrasive powders of decreasing particle size, fine machining, honing,etc.), there are also techniques which rely on the use of chemicaland/or electrolytic reactions. Thus, the technique is named “chemicalpolishing” when the reactions generated do not rely on an externalcurrent source and the technique named “electropolishing” when thereactions depend on an external current source, one of the electrodes(generally that connected to the positive pole of the electric currentsource) consisting of the workpiece to be polished.

The present invention falls within the technical context ofelectropolishing.

Electropolishing relies on two simultaneous and opposing reactions inwhich the relative rates and the diffusion phenomena at themetal/solution interface control the operating process. One of thesereactions is a dissolution reaction during which the metal passes intosolution in the form of ions; the other reaction is an oxidationreaction during which an oxide layer forms, this layer being protectiveto a greater or lesser extent and limiting, when it is present, theprogress of the first reaction. These two, opposing and complex,reactions come into competition with the consequence that the chemicaletching undergone by the metal surface is self-limiting, polishing beingmerely one particular result of this etching.

The polishing obtained electrolytically is substantially influenced bythe viscosity and/or resistivity of the electrolyte employed. It isknown to use various acid compositions, especially compositions based onhydrofluoric, sulfuric, nitric and phosphoric acids in variousconcentrations. Some of these acids (for example hydrofluoric acid)allow the oxide layer formed on the metal surface to be dissolved,whereas other acids (for example phosphoric acid, sulfuric acid, etc.)form the viscous medium necessary for the electropolishing to progress.It is essential for the concentrations of the constituents of theelectrolytes to be correctly controlled in order to ensure that theprocess progresses properly and to determine the lifetime of theseelectrolytes.

Many electropolishing bath compositions are known (see for example U.S.Pat. Nos. 3,766,030, 3,864,238, 5,591,320, 5,565,084, etc.). Some ofthese known compositions are multipurpose and allow both pure titaniumand its alloys to be treated. Because of this, the quality of action ofthese baths is the result of a compromise and polishing of the metalsurfaces treated is not optimal.

SUMMARY OF THE INVENTION

The object of the present invention is therefore essentially to providea bath composition for the specific electropolishing of nonalloyedtitanium so as to obtain a metal surface having a high-quality andmeasurable degree of polishing, but also so as to obtain, by anappropriate choice of the electrical parameters when using thecomposition, metal surfaces having a roughness which is predeterminable(“adjustable”) and measurable (for example in the case of biocompatiblebody implants made of titanium).

DETAILED DESCRIPTION OF THE INVENTION

For these purposes, a bath composition for the electropolishing of ametal surface made of nonalloyed titanium is characterized, inaccordance with the invention, in that it comprises:

sulfuric acid (95 to 98% solution): 20 to 40 vol %, this acid havingslight oxidizing properties and a high viscosity;

hydrofluoric acid (40 to 48% solution): 10 to 18 vol %, this acid givingrise to salts which are soluble; and

acetic acid (90 to 100% solution): 42 to 62 vol %, suitable formodifying the electrochemical equilibria at the solution/metalinterface, acetic acid allowing a better control of the oxidation andthe dissolution of the titanium surface and resulting in self-limitationof the chemical dissolution of the metal surface, the polishing of themetal surface being one of the results thereof.

The solution and concentration characteristics of the sulfuric andhydrofluoric acids are tailored to the type of metal to be polished(nonalloyed titanium).

None of the formulations known from the prior art employs acetic acidfor specifically polishing titanium. Acetic acid, owing to its chemicalproperties (low dissociation, etc.), allows a better regulation of theelectrochemical processes employed while the titanium is undergoingelectropolishing.

Advantageously, an addition agent called a “cationic wetting agent”, forexample a quaternary ammonium salt such as cetyltrimethylammoniumbromide or a substituted derivative such as hexadecylpyridinium bromide,in an amount ranging from 0.1 to 0.5 g/l, may also be added to theaforementioned bath composition. This agent modifies the polarization ofone of the two electrodes (alternate adsorption and desorptionphenomena) in the medium and leads to modifications in the double-layerphenomena. As a result, the quality of the polishing is improved andless metal is removed.

When employing the aforementioned bath composition, the followingconditions should be combined:

bath temperature between 20 and 22° C., so that the necessaryequilibrium between the rate of oxidation and the rate of dissolution ofthe oxide layer formed is not disturbed;

anode current density about 7 A/dm;

polishing electrical voltage (inter-electrode voltage) about 11 volts,these electrical characteristics (current density and voltage) beingtailored according to the shape of the surfaces to be polished and/or tothe possible use of one or more auxiliary anodes;

moderate stirring of the bath, this being able to be adapted for eachspecific application, so as to maintain the stability of the viscouslayer at the interface between the electrode (surface to be polished)and the liquid solution (a too vigorous or insufficient stirring woulddestabilize this interfacial layer and lead to poor polishing results),

whereby the rate of dissolution of titanium is about 6 microns/min.

Using the means proposed by the invention, it is possible to regulateand control extremely accurately the conditions of electrochemicaldissolution of the titanium metal surface and it is also possible toachieve a degree of polishing of titanium which is greatly superior tothat allowed by the techniques known hitherto. Thus, to give a specificcase, starting from an as-rolled titanium surface having a maximumroughness Rt of around 1 to 2 μm and an average roughness Ra of around0.1 to 0.15 μm, it is possible to obtain, after electropolishing underthe conditions of the invention, a maximum roughness Rt of around 0.5 μmand an average roughness Ra of around 0.05 to 0.10 μm with a dissolvedmetal thickness of around 50 to 100 pm. Furthermore, and above all, theconditions under which the electropolishing process are carried out arecompletely controllable so as to obtain a measurable and predeterminableroughness. Finally, the use of an addition agent as indicated abovemakes it possible, by better control of the conditions under which theprocess progresses, to remove a smaller thickness of metal in order toachieve a given roughness value.

One specific example of the abovementioned composition, without theaddition agent, is as follows:

sulfuric acid: 98% solution; density 1.84; 25 vol %;

hydrofluoric acid: 40% solution; density 1.10; 15 vol %;

glacial acetic acid: 100% solution; density 1.05; 60 vol %.

Roughness measurements taken on a nonalloyed titanium metal surface,before and after electropolishing, gave the following results(Rt=maximum roughness; Ra=average roughness): before polishing

before polishing Rt = 1.80 μm Ra = 0.176 μm (as - rolled surface): afterpolishing (metal Rt = 0.670 μm Ra = 0.080 μm thickness dissolved = 22μm): after polishing (metal Rt = 0.396 μm Ra = 0.057 μm thicknessdissolved = 59 μm): after polishing (metal Rt = 0.432 μm Ra = 0.080 μm.thickness dissolved = 116 μm):

What is claimed is:
 1. A bath composition for the electropolishing of ametal surface made of nonalloyed titanium, comprising: sulfuric acid: 95to 98% solutions, 20 to 40 vol %; hydrofluoric acid: 40 to 48% solution;10 to 19 vol %; and acetic acid: 90 to 100% solutions; 42 to 62 vol %,suitable for modifying the electrochemical equilibria at thesolution/metal interface, acetic acid allowing better control of theoxidation and the dissolution of the titanium surface and resulting inself-limitation of the chemical dissolution of the metal surface.
 2. Thecomposition as claimed in claim 1, comprising: sulfuric acid: 98%solution; density 1.84; 25 vol %; hydrofluoric acid: 40% solution;density 1.10; 15 vol %; glacial acetic acid: 100% solution; density1.05; 60 vol %.
 3. The composition as claimed in claim 1, furtherincluding an addition agent chosen from cetyltrimethylammonium bromideand hexadecylpridinium bromide, in an amount ranging from 0.1 to 0.5g/l.
 4. The composition as claimed in claim 2, further including anaddition agent chosen from cetyltrimethylammonium bromide andhexadecylpyridinium, in an amount ranging from 0.1 to 0.5 g/l.
 5. Amethod of using bath composition for the electropolishing of titanium,the bath composition comprising: sulfuric acid, 95-98% solution, 20-40vol %; hydrofluoric acid, 40-48% solution, 10-19vol %; and acetic acid,90-100%, solution, 42-62vol %, suitable for modifying electrochemicalequilibria at the solution/metal interface, acetic acid allowing bettercontrol of the oxidation and the dissolution of the titanium surface andresulting in self-limitation of the chemical dissolution of the metalsurface, the method comprising: providing a bath temperature betweenabout 20° C. and 22° C.; providing a current density of about 7 A/dm²;providing a polishing voltage of about 11 volts; and stirring the bathmoderately, whereby the rate of dissolution of the titanium is about 6microns/min.
 6. A method of using a bath composition for theelectropolishing of titanium, the bath composition comprising: sulfuricacid, 98% solution, density 1.84, 25 vol %; hydrofluoric acid, 40%solution, density 1.10, 15 vol %; and glacial acetic acid, 100%solution, density 1.05, 60 vol %, the bath composition comprising:providing a bath temperature between about 20° C. and 22° C.; providinga current density of about 7 A/dm²; providing a polishing voltage ofabout 11 volts; and stirring the bath moderately, whereby the rate ofdissolution of the titanium is about 6 microns/min.
 7. A method of usinga bath composition for the electropolishing of titanium, the bathcomposition comprising: sulfuric acid, 95-98% solution 20-40 vol %;hydrofluoric acid, 40-48% solution, 10-19 vol %; and acetic acid,90-100% solution. 42-62 vol %, suitable for modifying electrochemicalequilibria at this solution/metal interface, acetic acid allowing bettercontrol of the oxidation and the dissolution of the titanium surface andresulting in self-limitation of the chemical dissolution of the metalsurface, and an addition agent chosen from the cetyltrimethylammoniumbromide and hexadecylpyridinium bromide in amount ranging from 0.1-0.5g/l, the bath composition comprising: providing a bath temperaturebetween about 20° C. and 22° C.; providing a current density of about 7A/dm²; providing a polishing voltage of about 11 volts; and stirring thebath moderately whereby the rate of dissolution of the titanium is about6 microns/min.
 8. A method of using a bath composition for theelectropolishing of titanium, the bath composition comprising: sulfuricacid, 98% solution, density 1.84, 25 vol % ; hydrofluoric acid, 40%solution, density 1.10, 15 vol %; and glacial acetic acid, 100%solution, density 1.05, 60 vol %, the bath composition comprising:providing a bath temperature between about 20° C. and 22° C.; providinga current density of about 7 A/dm²; providing a polishing voltage ofabout 11 volts; and stirring the bath moderately, whereby the rate ofdissolution of the titanium is about 6 microns/min.